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Computer Systems and Hardware

Subject: Computer Science
Topic: 2
Cambridge Code: 0478

Computer Architecture​

Von Neumann Architecture - Foundation of modern computers

Key Components​

CPU (Central Processing Unit):

  • Executes instructions
  • Performs calculations
  • Controls all operations
  • Contains: Control unit, ALU, registers

Memory:

  • Stores data and programs
  • Fast, temporary storage
  • Lost when power off

Storage:

  • Permanent data storage
  • Slower but persistent
  • Survives power loss

I/O Devices:

  • Input: Keyboard, mouse, microphone
  • Output: Monitor, printer, speakers

The CPU (Central Processing Unit)​

CPU Functions​

Control Unit (CU):

  • Fetches instructions from memory
  • Decodes instructions
  • Controls data flow
  • Coordinates components

Arithmetic Logic Unit (ALU):

  • Performs arithmetic (add, subtract, multiply)
  • Performs logic operations (AND, OR, NOT)
  • Comparisons
  • Bit shifting

Registers:

  • Small, ultra-fast memory
  • Hold data being processed
  • Types: Accumulator (results), data, address

Fetch-Execute Cycle​

Step 1: Fetch

  • CU retrieves instruction from memory
  • Instruction loaded into instruction register

Step 2: Decode

  • CU interprets instruction
  • Determines required operation

Step 3: Execute

  • ALU performs operation if needed
  • Registers updated

Step 4: Store

  • Result stored in register or memory
  • Ready for next instruction

Clock Speed​

Clock frequency - Speed of fetch-execute cycle

  • Measured in GHz (gigahertz)
  • Higher frequency = more cycles/second
  • Faster execution (generally)
  • Generates heat, requires cooling

Memory Systems​

RAM (Random Access Memory)​

Volatile memory - Data lost on power loss

Characteristics:

  • Fast access
  • Main working memory
  • Temporary storage
  • Measured in GB

Types:

  • DRAM (Dynamic RAM) - Capacitors, refreshed constantly
  • SRAM (Static RAM) - Transistors, no refresh needed, faster, more expensive

Purpose:

  • Running programs
  • Storing data during execution
  • OS operations

ROM (Read Only Memory)​

Non-volatile memory - Data permanently stored

Characteristics:

  • Cannot be written to (normally)
  • Contains firmware
  • Survives power loss
  • Smaller capacity

Purpose:

  • BIOS (Basic Input/Output System)
  • Boot process
  • System initialization

Cache Memory​

Extremely fast memory between CPU and RAM

Levels:

  • L1 Cache: Smallest, fastest, CPU-integrated
  • L2 Cache: Larger, slightly slower
  • L3 Cache: Largest but still fast

Purpose:

  • Store frequently used data
  • Reduce main memory access
  • Improve performance significantly

Storage Devices​

Hard Disk Drive (HDD)​

Magnetic storage

  • Mechanical moving parts
  • Capacity: Terabytes (TB)
  • Speed: ~5400-7200 RPM
  • Reliability: 3-5 years typical
  • Cost: Cheap per GB
  • Advantage: Large capacity, inexpensive
  • Disadvantage: Slow, fragile, power consumption

Solid State Drive (SSD)​

Flash memory - No moving parts

  • Capacity: Terabytes (TB)
  • Speed: Very fast (no seek time)
  • Reliability: 10+ years
  • Cost: More expensive per GB
  • Advantage: Fast, durable, quiet, low power
  • Disadvantage: More expensive, temporary data loss risk

Optical Media​

CD/DVD/Blu-ray

  • Laser reads pits/lands on disc
  • Capacity: 700 MB (CD) to 50 GB (Dual-layer Blu-ray)
  • Speed: Slow, primarily for distribution
  • Reliability: Scratches cause data loss
  • Cost: Cheap but limited capacity

USB Flash Drives​

Portable storage

  • Flash memory (like SSD)
  • Capacity: Up to 1 TB
  • Speed: Fast, portable
  • Reliability: Good, compact
  • Purpose: Portable data transfer

Input/Output Devices​

Input Devices​

Keyboard:

  • Human-computer interface
  • Character/command entry
  • Scanning circuits for key detection

Mouse:

  • Pointing device
  • X-Y coordinate input
  • Optical tracking (modern)

Scanner:

  • Converts images to digital
  • Optical sensor reads colors
  • Software processes into files

Microphone:

  • Sound input
  • Analog-to-digital conversion
  • Audio recording

Output Devices​

Monitor:

  • Visual display
  • Resolution (pixel count)
  • Refresh rate (Hz)
  • Types: LCD, LED, OLED

Printer:

  • Hard copy output
  • Inkjet, laser, thermal
  • Color or monochrome

Speakers:

  • Audio output
  • Digital-to-analog conversion
  • Volume control

Processor Architecture​

Multi-core Processors​

Multiple cores - Several independent CPUs

  • Dual-core: 2 cores
  • Quad-core: 4 cores
  • Octa-core: 8 cores

Advantages:

  • Parallel processing
  • Better performance
  • Multitasking capability

Limitations:

  • Not all programs utilize multiple cores
  • Coordination overhead

Instruction Set​

ISA (Instruction Set Architecture) - CPU instruction types supported

Common types:

  • RISC (Reduced): Simple, fast (ARM)
  • CISC (Complex): Complex, flexible (x86)

Key Points​

  1. Von Neumann: CPU, memory, storage, I/O
  2. Fetch-Execute cycle: Fetch, Decode, Execute, Store
  3. RAM: Fast, volatile, temporary
  4. ROM: Non-volatile, firmware, boot
  5. Cache: Extremely fast, small capacity
  6. HDD: Large capacity, mechanical, slow
  7. SSD: Fast, reliable, expensive per GB
  8. I/O devices: Input (keyboard, mouse), Output (monitor, printer)

Practice Questions​

  1. Explain fetch-execute cycle
  2. Describe RAM vs ROM
  3. Compare HDD vs SSD
  4. Describe CPU components
  5. Explain cache importance
  6. List I/O devices
  7. Compare storage types

Revision Tips​

  • Know computer architecture clearly
  • Understand fetch-execute cycle
  • Learn memory hierarchy
  • Compare storage technologies
  • Know I/O device types
  • Understand CPU functions
  • Practice architecture diagrams